Physical and chemical kinetic processes in the CVD of silicon from SiH 2Cl 2/H 2 gaseous mixtures in a vertical cylindrical hot-wall reactor

Abstract

The kinetic process of the CVD of silicon is studied in the Si-H-Cl system on the basis of a large-scale experimental investigation of the growth rates. A cylindrical hot-wall LPCVD reactor was specifically built up and equipped with a sensitive microbalance. The physical transport phenomena are theoretically studied for a cylindrical geometry of both the hot reactional zone and the substrate itself: by solving the heat equation, a large isothermal area is found to extend around the substrate; the study of the momentum transfers reveals, by calculating gas velocities and streamlines, a very low disturbance of the gas flow by the occurrence of the substrate, due to a creeping laminar flow; at last, a coupled modelling of momentum and mass transfers shows, by computing gaseous species concentrations and deposition thicknesses profiles, that the growth rate is not influenced by total pressure, hardly by temperature, is increased by increasing the total flow rate and decreased by increasing the dilution ratio. Then, on the basis of thermodynamic approaches and considerations on adsorption phenomena, two theoretical mechanisms are proposed for the chemical process, depending on the experimental conditions. Taking into account theoretical and experimental kinetics, the temperature, the total flow rate and the total pressure are found to induce the transition between physical and chemical kinetic control. In both proposed chemical mechanisms, the limiting step is found to be the surface reaction between SiCl 2 adsorbed species and H 2 molecules. The predominant process is those with an activation energy of about 170 kJ mol −1 and a reaction order close to one with respect to H 2 species. The second mechanism, which involves an inhibition of the surface by atomic Cl species, occurs under more specifics conditions, i.e., high temperature, high dilution ratio and low total pressure.